This problem notably appears with a communication system such as the Automatic Identification system, also known by its acronym AIS. The AIS is a system of automated exchanges of messages between ships by radio which makes it possible to limit collisions between ships and which allows surveillance of maritime traffic in sight of coasts by means of coastal stations which listen to the exchanged communications. The collection of this data by satellite allows maritime surveillance far from coasts and a verification of the data transmitted by analysis of the received signal. The AIS messages, which are transmitted in a public manner, contain the position of the transmitting ship, as well as its identifier, its heading, its speed, etc.
When messages (or signals) are transmitted by ships sailing in dense zones such as the Mediterranean or along coasts, there is then a high probability of mutual blocking of signals on reception in the satellite. In fact, the workload of the input circuits of the receiver of a satellite is such that on average about twenty signals arrive simultaneously at the receiver, which makes their demodulation impossible.
In order to allow the satellite AIS to have acceptable performance, taking account of the performance requirements for maritime safety, it is necessary to limit said mutual blockages (that is to say the probability that several signals are received at the same time) or at least to limit their effect.
The currently examined solutions which make it possible to limit the collisions are of two orders:                modification of the system: one solution proposes modifying the transmitters of the ships in order to add a channel to them and defining a new protocol for transmitting a message which is more suited to spatial monitoring. This solution however has a high cost and a long period of deployment, and it could be bypassed by transmitting different messages on the conventional AIS channels and the Satellite AIS channels;        modification of the receiver: a large portion of the collisions of signals can be eliminated by using on-board receivers that are more complex both in terms of antenna systems and in terms of processing. This solution is however costly in payload complexity and most often in the complexity of the ground segment of the satellite task.        
Consequently, at present there is still a need for a system simultaneously satisfying all of said requirements, namely elimination of the effects of the collisions on the signals actually transmitted without a drastic increase in the complexity of the satellite payload.